Fe(Cp)2BF4: An efficient lewis acid catalyst for the aminolysis of epoxides

Article abstract of DOI:10.1055/s-0033-1340498

Ferrocenium tetrafluoroborate [Fe(Cp)₂BF₄] is an efficient Lewis acid catalyst for the aminolysis of aromatic, aliphatic, and cyclic epoxides using aniline and substituted anilines as the nucleophile to provide regioselective β-amino alcohols in 61-97% yields under solvent-free conditions at room temperature. The ring opening of cyclohexene oxide with aliphatic amines gave 2-aminocyclohexanols in 33-98% yields at 60 °C under solvent-free conditions. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1340498

PAPER
▌629
paper
Fe(Cp)2BF4: An Efficient Lewis Acid Catalyst for the Aminolysis of Epoxides
Ring Opening of Epoxides with Amines
Geeta Devi Yadav, ManMohan Singh Chauhan, Surendra Singh*
Department of Chemistry, University of Delhi, Delhi 110007, India
Fax +91(11)27666605; E-mail: ssingh1@chemistry.du.ac.in
Received: 18.09.2013; Accepted after revision: 09.12.2013
and Strecker reaction of ketones and aldehydes under sol-
Abstract: Ferrocenium tetrafluoroborate [Fe(Cp)2BF4] is an effi-
vent-free conditions at room temperature.22 Recently, en-
cient Lewis acid catalyst for the aminolysis of aromatic, aliphatic,
antioselective ring opening of meso-epoxides with aniline
and cyclic epoxides using aniline and substituted anilines as the nu-
cleophile to provide regioselective β-amino alcohols in 61–97%
using Bolm’s ligand and iron(II) perchlorate hexahydrate
yields under solvent-free conditions at room temperature. The ring was reported by Plancq and Ollevier.23 The better solubil-
opening of cyclohexene oxide with aliphatic amines gave 2-amino-
cyclohexanols in 33–98% yields at 60 °C under solvent-free condi-
tions.
ity of these organometallics in organic solvents and re-
agents may improve reactivity.
Key words: epoxides, ring opening, amino alcohols, Lewis acid,
Fe(Cp)2BF4
Fe+ BF4
PF6
Fe
Fe(Cp)2PF6
Fe(Cp)2BF4
β-Amino alcohols are useful synthetic intermediates in the
synthesis of a wide range of biologically active natural
and synthetic products, chiral auxiliaries, unnatural β-
amino acids, β-blockers in pharmaceuticals, and insecti-
cides.1 Ring opening of epoxides using excess amine as
the nucleophile at elevated temperatures is an important
and widely used route for the preparation of β-amino alco-
hols.2 However, this process is not suitable particularly
when dealing with thermally sensitive epoxides due to oc-
currence of side reactions.3 Lewis acids, many of which
suffer from deactivation of the catalyst due to complex
formation with the amine, have also been used for this
process. The cleavage of epoxides with amines has been
developed in presence of metal halides,4 metal triflates,5
metal alkoxides,6 metal amides and triflamide,7 transition-
metal salts,8 hexafluoropropan-2-ol under reflux,9 ionic
liquid,10 zirconium sulfophenyl phosphonate,11 montmo-
rillonite clay under microwave12a and solvent-free condi-
Figure 1 Structure of catalysts
The ring opening of cyclohexene oxide (1) with aniline
under solvent-free conditions at room temperature in the
presence of a catalytic amount (5 mol%) of various
iron(III) salts was examined as a model reaction (Table 1).
Reaction of 1 with iron(III) chloride monohydrate or
iron(III) sulfate monohydrate for 23 hours gave the de-
sired product, trans-2-anilinocyclohexanol (4a), in 16%
and 78% yields, respectively (entries 1 and 2); using fer-
rocenium tetrafluoroborate or hexafluorophosphate as the
catalyst for five hours at room temperature gave 4a in
90% and 73% yields, respectively (entries 3 and 4). Ferro-
cenium tetrafluoroborate (2 mol%) also gave 4a in a good
84% yield (entry 5). We examined the used of these two
catalysts, ferrocenium tetrafluoroborate and hexafluoro-
phosphate, in the ring opening of cyclohexene oxide (1)
with various substituted anilines 3b–e. Using 2-methoxy-
aniline (3b) as a nucleophile with ferrocenium tetrafluo-
roborate or hexafluorophosphate as the catalyst for five
hours gave the ring-opened product 4b in excellent yields
(96% and 84%) (entries 7 and 8). The results shown in Ta-
ble 1 indicate that ferrocenium tetrafluoroborate is a more
active catalyst than ferrocenium hexafluorophosphate. A
poor yield of product 4c was obtained with both catalysts
due to the lower solubility of solid 4-methoxyaniline in
the epoxide and the reaction requires dichloromethane as
a solvent to make the reaction mixture homogenous (en-
tries 9 and 10). Using ferrocenium tetrafluoroborate as the
catalyst and 2-methyl- and 4-methylaniline as nucleo-
philes gave ring-opened products 4d,e in 73% and 91%
yields, respectively, (entries 11 and 13).
tions,12b in water without
a
catalyst,13 silica,14
alumina/modified alumina,15 zeolites,16 Fe-MCM-41,17
SBA-15-pr-SO3H and Ti-MCM-41,18 heterodimetallic
coordination polymers,19 poly(amidoamine) dendrimer
supported on cross-linked polystyrene,20 and N-formyl-L-
proline.21 There are still limitations with some of the ex-
isting methods; for example less basic amines fail to open
these epoxides under ambient conditions or the use of
high catalyst loading using expensive catalysts.
Herein, we report our investigation into the ring opening
of epoxides with different amines using ferrocenium tet-
rafluoroborate (Figure 1) as a Lewis acid catalyst under
solvent-free conditions at room temperature. In our earlier
reports we used ferrocenium hexafluorophosphate as a
catalyst for the cyanosilylation of carbonyl compounds
We also studied the reaction of cyclopentene oxide (2)
with various substituted anilines using ferrocenium tetra-
fluoroborate (5 mol%) as the catalyst under solvent-free
conditions. We obtained an excellent yield of the products
SYNTHESIS 2014, 46, 0629–0634
Advanced online publication: 08.01.2013
DOI: 10.1055/s-0033-1340498; Art ID: SS-2013-Z0622-OP
© Georg Thieme Verlag Stuttgart · New York
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1
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G. D. Yadav et al.
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creasing the reaction temperature to 60 °C for six hours af-
forded the product in 96% yield ( entries 1 and 2). When
the same reaction was carried out in the absence of a cat-
alyst at 60 °C for six hours the product was obtained in
14% yield (entry 3).
Table 1 Ring Opening of Cyclic Epoxides with Different Anilines
Catalyzed by Iron(III) Salts under Solvent-Free Conditionsa
R2
H
N
R2
solvent-free
+
O
R1
We examined the use of various aliphatic amines, such as
isopropylamine, morpholine, benzylamine, (S)-α-methyl-
benzylamine, isobutylamine, ethylamine, and pyrrolidine,
for the ring opening of cyclohexene oxide (1) (Table 2).
The ring opening of cyclohexene oxide (1) with secondary
amines is faster than ring opening with primary amines
(entries 1–10). Ring opening of cyclohexene oxide (1)
with (S)-α-methylbenzylamine afforded trans-2-[(S)-1-
phenylethylamino)cyclohexanol in 95% yield with 58:42
diastereomeric ratio (entry 7).
R1
Fe(III) catalyst
n
OH
n
1 n = 2
2 n = 1
NH2
3a R1 = R2 = H
4a–e n = 2
5a–e n = 1
3b R1 = OMe, R2 = H
3c R1 = H, R2 = OMe
3d R1 = Me, R2 = H
3e R1 = H, R2 = Me
Entry Catalyst
Epoxide Amine
Product Yieldb
(%)
3a–f
1
2
FeCl3·H2O
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
3a
3a
3a
3a
3a
3a
3b
3b
3c
3c
3d
3d
3e
3e
3a
3b
3c
3d
3e
4a
4a
4a
4a
4a
4a
4b
4b
4c
4c
4d
4d
4e
4e
5a
5b
5c
5d
5e
16c
78c
90
73
84d
58d
96
84
61e
58e
73
64
91
73
95
90
97
92
90
Table 2 Ring Opening of Cyclohexene Oxide with Aliphatic
Amines Catalyzed by Ferrocenium Tetrafluoroborate under Solvent-
Free Conditionsa
Fe2(SO4)3·H2O
Fe(Cp)2BF4
Fe(Cp)2PF6
Fe(Cp)2BF4
Fe(Cp)2PF6
Fe(Cp)2BF4
Fe(Cp)2PF6
Fe(Cp)2BF4
Fe(Cp)2PF6
Fe(Cp)2BF4
Fe(Cp)2PF6
Fe(Cp)2BF4
Fe(Cp)2PF6
Fe(Cp)2BF4
Fe(Cp)2BF4
Fe(Cp)2BF4
Fe(Cp)2BF4
Fe(Cp)2BF4
3
RNH2/R2NH
Fe(Cp)2BF4
4
OH
O
5
neat, 60 °C
NHR/NR2
6
1
6
7
Entry
Amine
Time (h)
Yieldb (%)
8
1
2
3
6
24
6
96
77c
14d
9
N
H
10
11
12
13
14
15
16
17
18
19
4
5
9
6
92
95
N
H
O
N
H
H2N
H2N
Ph
Ph
6
7
21
20
98
95e
8
6
33
NH2
a Procedure: epoxide (2 mmol) was cooled to 0–5 °C, then catalyst (5
mol%) and amine (2.1 mmol) were added and the mixture was stirred
at 25 °C for 5 h.
9
6
77
85
H2N
H2N
10
19
b Yield was measured after purification by column chromatography.
c The reaction was stirred for 23 h.
a Procedure: cyclohexene oxide (1, 2 mmol) was cooled to 0–5 °C
then Fe(Cp)2BF4 (5 mol%) and amine (2.2 mmol) were added and the
mixture was stirred at 60 °C for the specified time.
b Isolated yield was measured after purification by column chroma-
tography.
d The reaction was carried using 2 mol% of catalyst.
e Reaction carried out in CH2Cl2 (0.5 mL).
5a–e (90–97%) with substituted anilines 3a–e in five
hours (entries 15–19).
c Reaction carried out at r.t.
d Reaction carried out in the absence of catalyst at 60 °C
e Diastereomeric ratio (58:42) of product was determined by gas chro-
matography.
The ring opening of cyclohexene oxide (1) with aliphatic
amines was also investigated using ferrocenium tetrafluo-
roborate (5 mol%) as the catalyst at 60 °C under solvent-
free conditions (Table 2). Ring opening of cyclohexene
oxide (1) with piperidine at room temperature for 24 hours
gave trans-2-piperidinocyclohexanol in 77% yield, in-
We conducted a competitive experiment for the ring-
opening of cyclohexene oxide (1) with aliphatic and aro-
Synthesis 2014, 46, 629–634
© Georg Thieme Verlag Stuttgart · New York

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Ring Opening of Epoxides with Amines
631
matic amines, piperidine and aniline, using ferrocenium (85:15) in 74% yield, but the dialkylated aniline 8b was
tetrafluoroborate (5 mol%) as the catalyst at room temper- also formed in 19% yield as a byproduct due to the reac-
ature and also at 60 °C for six hours (Scheme 1). The ma- tion of 8a and propylene oxide (entry 2). Performing this
jor product was that from the ring opening of cyclohexene reaction and monitoring it by gas chromatography gave
oxide (1) with piperidine and the minor product was that
from the opening of 1 with aniline, trans-2-anilinocyclo-
O
+
+
hexanol (4a), in a ratio of 96:4 at room temperature and
87:13 at 60 °C (product ratio was determined by gas chro-
matography).
N
H
NH2
Fe(Cp)2BF4
6 h
Furthermore, the range of application of this methodology
for ring opening of various epoxides was examined using
with aniline and ferrocenium tetrafluoroborate (5 mol%)
as the catalyst at room temperature (Table 3). The ring
opening of styrene oxide with aniline gave 2-anilino-2-
phenylethanol (7) in 82% yield by attack of the nucleo-
phile at benzylic carbon of styrene oxide. The product was
confirmed by 1H NMR and comparison with the reported
literature.4k Ring opening of propylene oxide with aniline
gave the desired product 8a and its regioisomeric product
OH
N
OH
+
N
H
at r.t.:
at 60 °C:
96%
87%
4%
13%
Scheme 1 Competitive ring opening of cyclohexene oxide with
piperidine and aniline
Table 3 Ring Opening of a Variety of Epoxides with Aniline Catalyzed by Ferrocenium Tetrafluoroborate under Solvent-Free Conditionsa
Entry
Epoxide
Time (h)
Product(s)
Yieldb (%)
Ph
NH
O
1
1
OH
82
Ph
Ph
7
OH
Ph
OH
H
OH
O
8a, 74c; 8b, 19
8a, 78c,d
N
2
3
7
5
Ph
N
8a
8b
OH
OH
Cl
OH
Cl
H
O
N
Cl
N
4
5
6
6
6
5
9a, 68; 9b, 23
Ph
Cl
Ph
9a
9b
OH
H
O
N
80e
92
H21C10
10
Ph
H21C10
OH
O
O
NHPh
O
11
12
OH
NHPh
O
7
4
97
a Conditions: epoxide (2 mmol) was cooled to 0–5 °C then Fe(Cp)2BF4 (5 mol%) and amine (2.1 mmol) were added and the mixture was stirred
at 25 °C for the specified time.
b Isolated yield was measured after purification using column chromatography.
c The regioisomeric product of 8a was obtained in 85:15 (by 1H NMR).
d The reaction was monitored by GC and the conversion of product 8a was 78% without formation of any byproduct.
e The regioisomeric product of 10 was obtained in 86:14 (by 1H NMR).
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 629–634

632
G. D. Yadav et al.
PAPER
••
H N
2••
BF4–
+
Fe
O
O
R
R
BF4–
BF4–
Fe+
+
Fe
BF4–
a
+
H
••
O
Fe
O
a
NH2
••
••
+N
b
••
H N
2••
R
H
R
(III)
(I)
BF4
b
HO
R
HN
Fe+
IV
H
O
NH
OH
H
R
N
+
R
V
(II)
Scheme 2 Proposed catalytic cycle for the ring opening of epoxide with aniline (a) R = alkyl, product IV is the major regioisomer and V is
the minor regioisomer; if R = Ph, V is the exclusive product
exclusively 8a in 78% after five hours with no byproduct. line to afford the corresponding β-amino alcohols in 68–
Ring opening of epichlorohydrin with aniline gave the de- 97% yield (1–7 h).
sired product 9a in 68% isolated yield and byproduct 9b
in 23% yield (entry 4). We also used aliphatic epoxides
1,2-epoxydodecane with aniline as the nucleophile, in this
case the major product 10 was obtained by attack of ani-
line at the terminal carbon of the aliphatic epoxide and a
minor regioisomeric product was also formed; the regio-
isomeric ratio was 86:14 and the total yield was 80%. The
ring opening of β-naphthylglycidyl ether for five hours
gave the desired product 11 in 92% yield (entry 7). 1,2-
Epoxycyclooct-5-ene also afforded trans-β-amino alco-
hol 12 in 97% yield in four hours.
1H and 13C NMR spectra were recorded on 400 MHz (operating fre-
quencies: 1H, 400.13 MHz; 13C, 100.61 MHz) Jeol FT-NMR spec-
trometers at r.t. using the NMR solvent as an internal reference
[CDCl3: δ = 7.26 (1H ) and 77.00 (13C)]. HRMS analysis was carried
out using Bruker QSTAR XL Pro system microTOF-Q-II. IR spec-
tra were recorded on a Perkin-Elmer FT-IR spectrophotometer. Op-
tical rotation values were measured on a Rudolph digital
polarimeter. TLC was carried out using Merck Kieselgel 60 F254
silica gel plates. Column chromatography separations were per-
formed using Merck Kieselgel 60 (Art. 7734). All new and known
compounds were characterized by 1H and 13C NMR and IR. The 1H
and 13C NMR data of known compounds 4a,4m 4b,11 4c,4m 4d,4g 4e,4f
5a,4m 5b,4k 5c,4m 5d,4g 5e,4f compounds from Table 2 (entries 1,4m
4,4m 5,4m 6,4i 724 and 105g), 7,4k 8a,8c 9a,8c 114k and 125e are similar
to those reported in the literature.
We proposed the mechanism for the ring opening of an
epoxide with an amine, according to the major regioiso-
mer formed during the reaction. The ferrocenium tetra-
fluoroborate acts as a Lewis acid and weak interaction is
possible with the oxygen of the epoxide. Nucleophile
amine can attack on both carbons of the epoxide, but the
formation of the major regioisomer depends on the nature
of epoxide and amine (Scheme 2). In the ring opening of
aliphatic epoxides with aniline attack occurs on the termi-
nal carbon (path b, intermediate II) of the epoxide and
give the major product IV and its regioisomeric minor
product V was also obtained due to attack of aniline at the
secondary carbon of the epoxide. The ring opening of sty-
rene oxide with aniline gave the product V by attack of the
nucleophile aniline at the benzylic carbon (path a, inter-
mediate III) of the epoxide.
β-Amino Alcohols; General Procedure
Catalyst Fe(Cp)2BF4 (5 mol%) was added to the cooled epoxide (2
mmol) at 0–5 °C and then amine (2.1 mmol) was added. The result-
ing mixture was stirred at 25 °C for the specified time (TLC moni-
toring). When the reaction was complete, the crude product was
purified by column chromatography. For entry 9 in Table 1, CH2Cl2
(0.5 mL) was used as solvent.
3,3′-(Phenylazanediyl)bis(propan-2-ol) (8b)
Brown liquid; yield: 81 mg (19%).
IR (CH2Cl2): 3318, 2967, 1598, 1504, 1375, 1083, 747, 694 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.22–7.19 (m, 2 H), 6.78–6.76 (m,
2 H), 6.54 (d, J = 8.05 Hz, 1 H), 4.16–4.08 (m, 2 H), 3.64 (dd, J =
15.38, 2.20 Hz, 1 H), 3.36 (dd, J = 14.64, 2.93 Hz, 1 H), 3.15 (dd,
J = 15.38, 9.52 Hz, 1 H), 3.00 (dd, J = 15.38, 9.52 Hz, 1 H), 2.01 (br
s, OH, 1 H), 1.17 (t, J = 5.86 Hz, 6 H).
13C NMR (100 MHz, CDCl3): δ = 149.14, 147.91, 129.18, 129.3,
117.41, 116.63, 114.01, 112.10, 66.02, 64.94, 62.45, 59.97, 20.15,
20.14.
In conclusion, we have demonstrated a new catalyst ferro-
cenium tetrafluoroborate for the ring opening of cyclic ep-
oxides with aliphatic and aromatic amines giving β-amino
alcohols in good and excellent yields in 5–20 hours under
solvent-free conditions. The catalyst was also used for the
ring opening of aliphatic and aromatic epoxides with ani-
HRMS (ESI): m/z [M + H]+ calcd for C12H20NO2: 210.1494; found:
210.1492.
Synthesis 2014, 46, 629–634
© Georg Thieme Verlag Stuttgart · New York

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Ring Opening of Epoxides with Amines
633
3,3′-(Phenylazanediyl)bis(1-chloropropan-2-ol) (9b)
Brown liquid; yield: 127 mg (23%).
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HRMS (ESI): m/z [M + H]+ calcd for C18H32NO: 278.2484; found:
278.2447.
trans-2-(sec-Butylamino)cyclohexanol (Table 2, Entry 8)
Colorless liquid; yield: 113 mg (33%).
IR (CH2Cl2): 3394, 2932, 2859, 1450, 1085, 847 cm–1.
1H NMR (400 MHz, CDCl3): δ = 3.34 (br s, 1 H), 3.03–3.01 (m, 1
H), 2.62–2.58 (m, 2 H), 2.16–2.13 (m, 1 H), 1.98–1.95 (m, 2 H),
1.62–1.58 (m, 2 H), 1.30–1.04 (m, 6 H), 0.97 (d, J = 6.59 Hz, 1.25
H), 0.91 (d, J = 6.59 Hz, 1.75 H), 0.85–0.77 (m, 3 H).
13C NMR (100 MHz, CDCl3): δ = 73.60, 60.36, 50.88, 32.96, 31.33,
30.78, 25.19, 24.20, 20.01, 10.36 (Major).73.82, 61.05, 51.27,
33.06, 30.90, 28.84, 25.19, 24.23, 21.27, 9.55 (minor).
HRMS (ESI): m/z [M + H]+ calcd for C10H22NO: 172.1701; found:
172.1722.
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trans-2-(Ethylamino)cyclohexanol (Table 2, Entry 9)
Brown liquid; yield: 110 mg (77%).
IR (CH2Cl2): 3390, 2932, 1450, 1084, 839 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 3.33–3.30 (m, 1 H), 2.87 (dt, J =
13.91, 10.98, 7.32 Hz, 1 H), 2.66–2.58 (m, 1 H), 2.44–2.39 (m, 1 H),
1.96–1.83 (m, 3 H), 1.63 (m, 2 H), 1.20–1.10 (m, 6 H).
13C NMR (100 MHz, CDCl3): δ = 71.66, 62.76, 40.18, 33.81, 28.23,
24.36, 24.03, 13.30.
HRMS (ESI): m/z [M + H]+ calcd for C8H18NO: 144.1388; found:
144.1392.
Acknowledgment
S.S. acknowledges the University of Delhi for financial assistance
and University Scientific Instrumentation Center (USIC), Universi-
ty of Delhi, India for analytical data. G.D.Y. thanks CSIR for pro-
viding a Junior Research Fellowship and M.S.C. is grateful to UGC
for providing a Junior Research Fellowship.
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